Friction charging is one cause of static electricity, generated by friction between two subjects, and peeling charging is another cause of static electricity, generated by separation of a closely adhered subject. Static electricity can cause the adhesion of foreign materials, such as dust, electrostatic breakdown of a device, malfunctioning of a measuring instrument, and fire. According to the widespread use of mobile computers, liquid crystal TVs, and multifunctional cell-phones, liquid crystal displays are in increasing demand and thus the inhibition of static electricity development caused by the integration of each auxiliary device has become a major concern.
According to the increasing demand for liquid crystal displays (LCD), optical films such as polarizing plates are also in demand, thus requiring high speed LCD production processes. Major LCD production companies have been focused on the production of small panels up to 20 inches, but recently, according to new techniques, the sizes of products has become larger than 20 inches.
Various treatments have been performed on the surface of polarizing plates. For example, the surface of a polarizing plate can be coated with an anti-glare layer, a low-refractive and/or high-refractive layer, and an anti-staining layer, and each layer has a different surface roughness and surface energy. In general, to produce the anti-glare layer, micro-sized inorganic particles are mixed with acrylic resin as a binder, followed by coating. At this time, the popular inorganic particle is micro-sized silica. So, the surface roughness of the anti-glare layer is coarser than that of the triacetyl cellulose layer itself. Thus, wettability of an adhesive applied to a protective film is very important. In the meantime, using acrylic resin binder induces hydrophobicity because of lowered surface energy, compared with the triacetyl cellulose layer.
With the realization of high-speed polarizing plate production, TFT IC device destruction has been observed during the peeling of a polarizing plate protective film, which is attributed to static electricity generation not common in the conventional processes, resulting in a poor LCD panel. It is known that TFT IC device destruction has been observed when at least 0.7 kV of electrostatic voltage is generated on the surface of a polarizing plate.
To avoid such a generation of static electricity, an anti-static layer has been applied on the outside of a polarizing plate or a protective layer, but the effect is controversial and static electricity development has not been controlled yet. Therefore, an anti-static function needs to be added to an adhesive layer to inhibit static electricity development from the beginning.
The conventional methods to add an anti-static function onto an adhesive layer are adding a compound having a conductive component such as conductive metal powder or carbon particles, and adding an ionic or non-ionic material in the form of a surfactant. In the case of adding the conductive metal powder or carbon particles to give an anti-static function, the required amount of such a compound is huge, resulting in a decrease in transparency. In the case of adding an ionic or non-ionic material in the form of a surfactant, the composition is affected easily by humidity and the adhesive property is reduced by bleeding into the surface of the adhesive.
Another conventional technique of eliminating static electricity is by using a static eliminator during the protective film separation process. This technique also has a problem of lowered productivity owing to the limitation of static elimination and slow processing speed.
Korean Patent Publication No. 2004-0030919 describes an organic salt being added by at least 5 weight % to give a pressure sensitive adhesive and an anti-static effect is brought about by inducing a specific resistance up to 1013 Ω/□ on the adhesive surface. However, according to this method, high priced organic salt has to be used and only surface resistance is reduced without predicting the variations of constant voltage caused by static electricity generated during the separation.
Japanese Laid-Open Patent Publication No.2004-287199 describes an ionic conductive polymer having a hydroxyl group being added to endow an anti-static function. However, this polymer is reacted with a conventional cross-linking agent (isocyanate), so that the adhesive property and the rheological property are changed, and accordingly the anti-static and adhesive capacities are out of control.
Japanese Laid-Open Patent Publication No. Hei6-128539 describes a method to generate an anti-static function by mixing a polyether polyol compound and one or more alkali metal salts. However, if the cross-linking agent used is isocyanate, the degree of cross-linking is vulnerable and the adhesive property is reduced by surface migration attributed to an ether bond (that is attributed to the hydrophilicity of ethylene oxide).